Fresh install of RC, new user dir.
If you go to main menu Tools|Options, Editor tab and Code Completion, there is a list of languages. Choose HTML and uncheck "Auto Popup Completion Window". Press OK. Now you can go to the same menu to verify, that it is unchecked. Now check it back (enable it again) and press OK. Go to the same menu, this choice is unchecked. I repeated it several times and I didn't find a way to make it checked. Restarting IDE doesn't help.
Product Version: NetBeans IDE 7.0 RC1 (Build 201103150000)
Java: 1.6.0_24; Java HotSpot(TM) Client VM 19.1-b02
System: Windows XP version 5.1 running on x86; Cp1250; cs_CZ (nb)

fixed in web-main#817ad86bbec8
Since the user is not able to normally reenable the feature this should be fixed in 7.0. The fix just corrects a kind of typo so it is 100% safe.
Vladimire, please verify. Thanks.